1. Field of the Invention
The present invention relates to a so-called "torch ignition type internal combustion engine" having a precombustion chamber for receiving an air-fuel mixture which is initially ignited by a spark plug to form a torch jet by means of which an air-fuel mixture in a main combustion chamber is ignited to generate power. More particularly, the invention relates to an internal combustion engine of the mentioned class in combination with an improved spark plug.
2. Description of the Prior Art
The conventional spark ignition type internal combustion engine generally has a combustion chamber in which an air-fuel mixture is ignited by a spark plug. The emission of harmful components of engine exhaust gases has recently been a public problem. In an attempt to solve the problem, researches have been made to operate engines with lean air-fuel mixtures in order that the harmful exhaust components might be reduced. As a result of the researches, there has been developed the so-called "torch ignition type" internal combustion engine which has a main combustion chamber defined by a cylinder in a cylinder block, a piston in the cylinder and a cylinder head mounted on the cylinder block. A precombustion chamber is provided separately of the main combustion chamber but communicated therewith by a passage. The main combustion chamber and the precombustion chamber are supplied with charges of air-fuel mixtures which may be of either the same or different air-fuel ratio. The mixture charge in the precombustion chamber is first ignited by a spark plug to form a flame which is called "torch jet" which runs through the passage into the main combustion chamber to ignite the air-fuel mixture charge therein. The air-fuel mixture charges are sufficiently lean as a whole to reduce the harmful components of the engine exhaust gases.
The spark plug used in this kind of internal combustion engine is so mounted on the engine that the spark producing section of the plug is positioned near to the passage so as to eliminate the reduction of the ignitability of the spark plug which would otherwise be caused by the residual gases remaining in the precombustion chamber. This positioning of the spark producing section of the spark plug, however, tends to cause the electrodes and, particularly, the grounded electrode, to be overheated. In order to prevent the overheat, therefore, the spark plug used is of a high heat value and of a design that the grounded electrode is shorter compared with the ordinary spark plug to avoid the overheat of the grounded electrode as much as possible. Because of this design, the end of a tubular metal shell to which the grounded electrode is connected extends axially beyond the end of an insulator supporting the central electrode.
With the spark plug of the discussed design, when the engine is operated in such a condition that an air-fuel mixture in the precombustion chamber produces a sooty carbon, the carbon is deposited on the insulator around the central electrode. Because the spark plug is of the high heat value and the transfer of the heat from the spark producing section of the plug to the cylinder head is good, the insulator around the center electrode is at a temperature which is not high enough to burn away the deposit of carbon when the engine is running at an ordinary or coasting speed. The deposition of carbon on the insulator around the center electrode is thus increased, so that the insulation resistance of the insulator around the center electrode is reduced, with a result that a misfire is caused.